DC-to-DC converters have been widely applied in various electronic circuits, with high-side and low-side switches in a switching circuit controlled by a control circuit to generate a stable output voltage for a supply voltage to its load circuit. FIG. 1 shows a circuit diagram of a conventional switching circuit 10 for an N-phase DC-to-DC converter, which comprises N power stages 12 coupled in parallel between two power nodes, i.e., power supply Vin and ground GND, and each of them including a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) 122 serving as the high-side switch coupled between the input voltage Vi and a phase node PHASEn and a MOSFET 124 serving as the low-side switch coupled between the phase node PHASEn and ground GND, and the MOSFETs 122 and 124 are switched by control signals UGn and LGn at their gates 1226 and 1246, respectively, to regulate a current ILn flowing through an inductor Ln to charge a capacitor Co to produce the output voltage Vout of the converter, where n=1, 2, 3, . . . , N. FIG. 2 shows a connection layout on a printed circuit board (PCB) for the MOSFETs 122 and 124 of one power stage 12, on which the metal strips are designated with the respective element numerals of the circuit 10 to be bonded thereon. In a PCB, the power nodes of input voltage Vin and ground GND have large metal planes (not shown in FIG. 2), and thus they may enhance heat dissipation. The phase node 126 has a small metal plane since it is a signal node, and it almost does nothing for heat dissipation. A MOSFET can endure high voltage and has good capability to dissipate heat at the drain side, but cannot endure high voltage and has poor capability to dissipate heat at the source side. Therefore, the high-side MOSFET 122 can dissipate heat to the power node Vin through its drain side, and thus it has good heat dissipation capability. The low-side MOSFET 124 can dissipate heat to the power node GND through its source side, but the heat dissipation capability is poor at the source side. To prevent the low-side MOSFET 124 from over-heated to result in improper operations, all the low-side MOSFETs 124 of the switching circuit 10 are individually packaged one by one, and thus they will occupy large PCB layout area.
Therefore, a switching circuit having good heat dissipation capability and small PCB layout area is desired.